The proposed research deals with the control of head movements by the vestibular system. Such movements play an important role in postural reflexes and the orientation of cranial sense organs, but we do not know how these movements are organized within the brainstem and cervical spinal cord. Abnormalities in vestibulocollic function in man are associated with deficits in postural regulation and are common sequelae of clinical conditions such as Meniere's disease. To understand vestibular control of head movements we need to know: (1) the functional components of the vestibular nerve, and (2) how these are related to the various classes of vestibulocervical neurons. In experiments already completed or underway I am characterizing the classes of vestibulospinal neurons which relay information to neck motorpools in Pseudemys scripta. The proposed research will extend this analysis of head muscle control. (1) Peripheral axons of vestibular primary afferents will be labelled with horseradish peroxidase (HRP) and characterized in terms of 4 morphological parameters that are known to have functional significance: diameter, and terminal type, morphology, and location on the crista. (2) Hair cells afferent to vestibular primaries will be examined with light and electronmicroscopy, and an hypothesis will be tested that relates spatial variation in hair bundle structure to known physiological differences between primaries. (3) Labelled central axons of vestibular primaries will be characterized in terms of their diameter, branching pattern and relations to vestibular nucleus neurons. (4) Quantitative techniques will be used to identify classes of peripheral and central axons based on their morphologies, afferents, and central targets, and to determine probable pairings of peripheral with central axon classes. The object of these experiments is to identify the various morphological population of vestibular primary afferents and to help understand the functional role each plays in the organization of head movements. This research, combined with work in progress, will contribute to our understanding of movement control in two ways. The proposed research will constitute the first comprehensive, systematic morphological description of information channels in the vestibular nerve of any vertebrate. Thus it will contribute to a general understanding of vestibular information processing. Second, the proposed research will provide important data on mechanisms underlying the descending control of axial motor systems.